Cocalc uses a websocket and xterm.js to implement terminals on a remote server. Each terminal session corresponds to a file (with extension .term), so multiple clients can open the same session by opening the same file. If you type in one session then all sessions will see the typing at the same time. (Disclaimer: I wrote this. It’s way too heavy for this use case, but might be an amusing demo or proof of concept for somebody to play with before writing something new.)